Filter apparatus of the type to which the present invention relates is shown and described in U.S. Pat. Nos. 3,087,620 and 3,091,336. The filter apparatus comprises generally a tank into which the liquid suspension is continuously introduced and a vacuum chamber located beneath the tank and separated therefrom by a perforate rigid septum over which a flexible filter sheet is movably disposed. The suspended solid is collected or deposited on one face of the sheet in the form of a cake and clarified liquid flows into the vacuum chamber as a vacuum is applied thereto.
It has been common practice to use a disposable filter sheet which is supported on a roll at one end of the filter unit and periodically index the sheet along the rigid septum by a power actuated conveyor. The conveyor includes spaced chains which engage the marginal edges of the sheet and flights disposed between the chain which engage the filter sheet laterally thereof along lengthwise spaced intervals. The conveyor is indexed at times dictated by the load of suspended solid deposited on the filter sheet. The frictional engagement of the chain and flights causes the filter sheet to be displaced lengthwise on the septum. The filter sheet with the suspended solid deposited thereon is then discarded at the discharge end of the unit.
Efforts have been made to repetitively use the filter media rather than discarding it after a single use. In such arrangements the filter media is arranged as an endless loop. The loop filter media is located in the filter unit with a length thereof disposed beneath the conveyor chains and flights as with the single use or discardable filter sheet or media. The loop is separated from engagement with the conveyor at the filter discharge end. Cleaning and suspended solid or cake removing means are provided at the discharge end for removing the deposited solids/cake from the filter media and for cleaning as by spraying. The filter media loop re-engages the septum at the forward end of the tank and is guided beneath the conveyor chains and flights.
In another embodiment of filter loop attachment, the loop has been fastened directly to the chain rather than using friction alone to accomplish the indexing movement.
During indexing movements, the vacuum to the vacuum chamber is momentarily cut-off so that the filter media is released from its normal gripping engagement with the perforate septum. This permits the filter sheet to be moved more easily along the face of the septum. Under some conditions, however, vacuum cut-off causes a backflow of the filtrate upwardly through the perforate septum. Such backflow causes the filter sheet to bow away from the septum and separate from engagement beneath the chain. This allows the unfiltered suspended solid to flow directly into the chamber and subsequently to its reuse point. This condition materially reduces the efficiency of the filter unit.